Metal particles can be synthesized, for example, by a gas phase method, in which metal vapor evaporated at high temperature is supplied into a gas phase to cool the vapor rapidly by collision with gas molecules, thereby forming fine particles; a solution method, which may be referred to as a liquid phase method hereinafter, in which a reducing agent is added to a solution in which metal ions are dissolved to reduce the metal ions, or some other method.
Metal hydroxide fine particles can be synthesized, for example, by the above gas phase method; or a liquid phase method, such as a solution method of controlling pH or anions of an aqueous solution in which metal ions are dissolved, to take out a metal hydroxide, and subsequently drying or firing the hydroxide, as well as a sol-gel method, a reversed micelle method, or a hot soap method.
Among these methods, the liquid phase method has an advantage that the particles can be relatively inexpensively synthesized in great volume. The liquid phase method is usually performed by adding a metal cation solution, and a reducing agent solution or a solution that contains hydroxide ions, to a reaction vessel having a stirrer. By initial addition thereof, nuclei are formed, and by subsequent addition, crystal growth is caused. For example, various methods used to form silver halide grains are known (see, for example, JP-A-7-219092 (“JP-A” means unexamined published Japanese patent application), JP-A-8-171156, JP-A-4-283741, JP-B-8-22739 (“JP-B” means examined Japanese patent publication), and U.S. Pat. No. 3,782,954). However, when any of these methods is used in nuclei formation, any stirring that can be used makes the liquid circulate in the reaction vessel, and therefore the nucleus formation is caused in parallel to nucleus growth, and as a result, it is difficult to form monodispersive nuclei.
To perform mixing of the added liquid without mechanical stirring, methods wherein no circulation of the added liquid is performed are also disclosed (see, for example, JP-A-4-139440 and JP-T-6-507255 (“JP-T” means searched and published International patent application)). However, in these methods, the power of the mixing is insufficient, since intense stirring is not performed. Methods of mixing the two liquids in a pipe are also disclosed (see, for example, U.S. Pat. No. 5,104,786, and JP-A-11-38539). Although no circulation of the added liquid is generated in this case, the added liquid is so-called plug flow, which flows in a constant direction; therefore, it is unavoidable that the mixing relies on the generation of turbulence accompanying high flow velocity. Thus, to generate sufficient turbulence in the plug flow, it is necessary to maintain a very high velocity flow, and carrying out this involves difficulty.
To keep sufficient mixing power without any mechanical stirring, there are known methods to make the added liquid into a linear jet flow, and perform the mixing by kinetic energy thereof. For example, a method of using kinetic energy of such a jet flow to produce a silver halide photographic emulsion is also disclosed (see, for example, JP-A-8-334848). However, the method disclosed in JP-A-8-334848 is a production method based on a single jet method; mechanical stirring is together used, since the used kinetic energy is insufficient to perform mixing in the entire reaction vessel.
There are also known methods to make at least one of two-type aqueous solutions into a linear jet flow having a high flow velocity, and mix the two-type solutions in a short period of time, to produce silver halide grains continuously (see, for example, JP-A-2000-338620 and JP-A2001-290231). However, even if high flow velocity is used in these methods, the caused mixing is insufficient for mixing microscopically. Thus, further improvement has been desired.
In the dispersion obtained by the reaction, a salt(s) or a decomposition product(s), which are by-products, are dissolved together with metal hydroxide fine particles. Thus, it is usually necessary to remove these by-products. To remove the salts or the decomposition products, for example, ultrafiltration, electrodialysis, or centrifugal separation is used. However, when the dispersion containing fine particles of nanometer size is treated, the former two methods cause the filtration membrane or the dialysis membrane to be easily clogged, and as such the methods are not practical. The latter method is inefficient, since batch treatment must be conducted.